DESCRIPTION

These functions raise and lower the system priority level. They are used
by kernel code to block interrupts with priority less than or equal to
the named level (i.e., spltty() blocks interrupts of priority less than
or equal to IPL_TTY). The code may then safely access variables and data
structures which are used by kernel code that runs at an equal or lower
priority level.
An spl function exists for each distinct priority level which can exist
in the system. These macros and the corresponding priority levels are
used for various defined purposes, and may be divided into two main
types: hard and soft. Hard interrupts are generated by hardware devices,
while soft interrupts are generated by callouts and called from the
kernel's periodic timer interrupt service routine.
In order of highest to lowest priority, the priority-raising macros are:
splhigh() blocks all hard and soft interrupts. It is used for code
that cannot tolerate any interrupts, like hardware con-
text switching code and the ddb(4) in-kernel debugger.
splserial() blocks hard interrupts from serial interfaces. Code run-
ning at this level may not access the tty subsystem.
splsched() blocks interrupts that may access scheduler data struc-
tures. Code running at or above this level may not call
sleep(), tsleep(), or wakeup(), nor may it post signals.
Note that "running" means invoked by an interrupt handler
that operates at this level or higher. Kernel code that
operates in the context of a process and has called
splhigh() for blocking purposes can use sleep(),
tsleep(), or wakeup().
splclock() blocks the hardware clock interrupt. It is used by hard-clock() to update kernel and process times, and must be
used by any other code that accesses time-related data.
splstatclock() blocks the hardware statistics clock interrupt. It is
used by statclock() to update kernel profiling and other
statistics, and must be used by any code that accesses
that data. This level is identical to splclock() if there
is no separate statistics clock.
splvm() blocks hard interrupts from all devices that are allowed
to use the kernel malloc(9). That includes all disk, net-
work, and tty device interrupts.
spltty() blocks hard interrupts from TTY devices.
splsofttty() blocks soft interrupts generated by serial devices.
splnet() blocks hard interrupts from network interfaces.
splbio() blocks hard interrupts from disks and other mass-storage
devices.
splsoftnet() blocks soft network interrupts.
splsoftclock() blocks soft clock interrupts.
Two macros lower the system priority level. They are:
spllowersoftclock() unblocks all interrupts but the soft clock inter-
rupt.
spl0() unblocks all interrupts.
The splx() macro restores the system priority level to the one encoded in
s, which must be a value previously returned by one of the other spl mac-
ros.
The splassert() function checks that the system is running at a certain
priority level. The argument s should be one of these constants:
IPL_STATCLOCK
IPL_CLOCK
IPL_VM
IPL_BIO
IPL_TTY
IPL_NET
IPL_SOFTNET
IPL_SOFTCLOCK
IPL_NONE
The splassert() function is optional and is not necessarily implemented
on all architectures nor enabled in all kernel configurations. It checks
the current system priority level to see if it's at least at the level
specified in the argument s. If possible, it also checks if it hasn't
been called from an interrupt handler with a level higher than the one
requested, which must be an error (if some code is protected from
IPL_SOFTNET interrupts, but accessed from an IPL_NET interrupt, it must
be a design error in the code).
The behavior of the splassert() function is controlled by the
kern.splassert sysctl(8). Valid values for it are:
0 disable error checking
1 print a message if an error is detected
2 print a message and a stack trace if possible
3 like 2 but also drop into the kernel debugger
Any other value causes a system panic on errors.
MirOS BSD #10-current March 11, 1997 2